The present invention relates to a polycentric variable axis hinge for use. in particular, but not exclusively, in conjunction with orthopedic knee braces of the type intended to reinforce an injured knee joint. The hinge of the present invention constitutes an improvement over the hinge described and claimed in U.S. Pat. No. 5,230,696 which issued Jul. 27, 1993.
As pointed out in that patent, the human knee is acknowledged as one of the weakest joints in the body. It is the articulating joint between the thigh and calf muscle groups, and it supports the weight of the body while a person is walking or running. The joint is held together by two small but strong ligaments, namely, the anterior and posterior cruciate ligaments. Knee instability arising out of cartilage damage, the ligament strain and other causes is relatively commonplace since the knee joint is subjected to significant loads during the course of almost any kind of physical activity requiring the use of the legs.
The thigh and legs of the human body are joined to each other through the knee joint, and the principal motions of the knee joint are extension and flexion due to rotation about a horizontal axis extending across the knee joint in a medial-lateral direction. These motions are complex because they take place about a rotational axis which is not fixed, as is the case with the elbow, but which shifts slightly across the knee joint in the anterior-posterior direction. It is important for an adequate knee brace to follow the shifting path of the rotational axis of the knee as closely as possible, and accordingly the principal objective of the present invention is to provide a knee brace hinge which is capable of fulfilling such a criterion.
Additionally, and particularly in the case of injured or weak knees, means must be provided in the knee brace hinge to control the extension of the knee joint and to stop the extension, for example, short of 15%-5% of full extension, in order to prevent knee ligament injuries. Another objective of the invention is to provide a knee brace hinge which incorporates improved stop means that may be adjustable to tailor ,the hinge to individuals, to control the extension of the knee joint, and to stop the extension at a predetermined degree of full extension.
In U.S. Pat. No. 4,773,404, and in its parent U.S. Pat. No. 4,723,539, and in U.S. Pat. No. 4,890,607, multi-axis controlled motion knee hinges are disclosed. Each hinge is constructed to enable the tibia to slide rearwardly relative to the femur for a predetermined distance throughout an initial range of flexion of the knee from a straight leg position, and beyond that initial range of flexion, to rotate relative to the femur along a predetermined arcuate path.
In the construction of the hinge described in U.S. Pat. No. 4,890,607, the end portions of femur and tibia are interconnected by a cam assembly comprised of a pair of cam slots in one of the links of the hinge, and cam follower pins mounted on the other link and engaging the slots. In the particular construction described in U.S. Pat. No. 4,890,607, each slot has a straight segment adjoining an arcuate segment of equal radius.
The variable axis hinge of the present invention is of the same general type as the hinge disclosed in U.S. Pat. No. 5,230,696 and in U.S. Pat. No. 4,890,607, but it is constructed in a manner such that the hinge more closely follows the actual movement of the knee, as the leg is bent in the rearward direction.
The hinges described in U.S. Pat. Nos. 5,230,696 and 4,890,607 each comprises a knee brace hinge mechanism which utilizes first and second variable radii camming slots and corresponding cam pin followers. One of the camming slots serves to provide the anterior motion of the upper link of the hinge, while the second camming slot provides for the unicentric phase of the hinge artarokinematics. During the initial range of motion, pivoting occurs through a short multi-radii slot segment about one of the cam pin followers disposed within that slot. After that cam pin follower reaches the anterior end of the slot, it serves as a pivot for movement of the other cam pin followers along the second variable radii slot.
When hinges of the type being discussed are used in conjunction with orthopedic knee braces, they provide a polycentric mechanical joint between the femoral and tibial cuffs of the brace. For that application, the hinge includes a femoral stem which is attached to the femoral cuff and which extends downwardly from the femoral cuff, and it also includes a tibial stem attached to the tibial cuff and which extends upwardly from the tibial cuff. The action of the hinge is such that angular movements of the stems relative to one another cause them to slide and rotate relative to one another so as to replicate the actual movement of the knee joint of the user as the leg is bent and straightened.
The hinge of the present invention is constructed in a manner such that the femoral and tibial stems are angulated so as to position the axes of rotation of the hinge of the order of two-thirds of the distance from the patella to the rear of the patient's knee in order to locate the axes of rotation of the hinge in a position in which the hinge precisely simulates the anatomical femoral arc in the human knee.
The hinge of the invention is also constructed to be relatively thin as compared with the hinge described in U.S. Pat. No. 5,230,696, and it is rugged in its construction and relatively light and comfortable insofar as the patient is concerned.
Accordingly, a general objective of the present invention is to provide an improved multi-axis hinge for an orthopedic knee brace which is constructed to have the ability accurately to replicate the complex movements of the knee of the patient, as the leg is bent and straightened.
Another objective of the invention is to provide such a multi-axis hinge which is light in weight and which has relatively high integral strength, as well as tolerance precision and production efficiency.
Yet another objective of the invention is to provide such a variable axis orthopedic knee brace hinge which is constructed in a manner so as to render the knee brace in which it is mounted comfortable to the wearer.